merge: match the help text with the documentation

We used to be very casual in terminology and used <branch>, <ref> and
<rev> more or less interchangeably with <commit>.  Match the help text
given by "git merge -h" with that of the documentation.
Signed-off-by: NJared Hance <jaredhance@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -37,8 +37,9 @@ struct strategy {
 };
 
 static const char * const builtin_merge_usage[] = {
-	"git merge [options] <remote>...",
-	"git merge [options] <msg> HEAD <remote>",
+	"git merge [options] <commit>...",
+	"git merge [options] <msg> HEAD <commit>",
+	"git merge --abort",
 	NULL
 };